Photoresists capable of forming a super-fine pattern with excellent resolving power and good rectangular section have been developed in rapid succession for the purpose of manufacturing ultra-large scale integration 4M-bit or 16M-bit semiconductor elements. These photoresists have been designed for optical exposure techniques using g line or i line. Materials for forming such photoresists contain a novolak resin and a naphthoquinonediazide compound as the fundamental constituents. It is well known that the solubility of these photoresists in alkali is decelerated in an unexposed state by the inhibition effect due to mutual action between the fundamental constituents and is accelerated in a light-struck state by carboxylic acid produced from the photolysis of the naphthoquinonediazide compound.
It is widely accepted that micropattern formation can be accomplished using photo lithography and an appropriate resist. However, devices capable of producing super-fine dimensions of less than a half-micron are considered impossible using conventional exposure techniques with a g line or i line regardless of advances in stepper apparatus technology (for example, increase in NA value or improvement in manufacturing processes) because these techniques are approaching the limit of the wavelength of light. Considering current manufacturing techniques, however, it appears that micropattern formation by light exposure techniques will remain important. A light source that has attracted attention to replace the g line or i line is deep UV, particularly from an excimer laser of KrF, ArF or the like because it offers a solution to the problem of wavelength. In order to use deep UV, many problems must be overcome. In particular, at present there are no resists adapted for use with deep UV such as produced by an excimer laser.
When an existing g line resist is used as a resist for an excimer laser, a triangular pattern profile results instead of a rectangular pattern profile. This results because both the novolak resin and the light-sensitive substance that constitute a g line resist show strong absorption around the wavelength of 250 nm. Thus, the resist absorbs the incident light so much that the incident light fails to fully reach the substrate surface. This results in an exposure gradient in the thickness of the resist. Because of this, commercially available resists provide a rectangular profile only when their thickness is 0.5 .mu.m or less. In addition, even when the upper portion of the pattern has a rectangular profile, the lower portion thereof fans out toward the bottom, which makes them impractical for micropatterns.
Attempts to solve this problem have involved combining a light-sensitive substance showing good bleachability with a polymer showing less absorption around the wavelength of the exposing light (for example, a combination of an imide-containing polymer and a photoacid as described in JP-A-62-229242 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") which corresponds to U.S. Pat. No. 4,837,124; a combination of a light-sensitive substance having a linkage of ##STR1## and a less absorbing polymer as described in JP-A-62-235935; and a combination of a light-sensitive substance having a linkage of ##STR2## and a less absorbing polymer described in JP-A-63-240543).
An example of the less absorbing polymer is a phenol-cresol novolak resin such as an o-chloro-m-cresol resin and a polymer having the structure of: ##STR3##
In addition, JP-A-61-126549 discloses obtaining a pattern having an improved profile by using a dilute alkali developer under controlled developing conditions.
In general, it is considered important for resists for excimer laser lithography to ideally satisfy the following three requirements:
(1) have good plasma resistance;
(2) be developable with an alkali solution; and
(3) have high resolving power and form a rectangular profile.
Materials that satisfy all these requirements have yet to be found.
The m/p-cresol novolak resins are widely used as constituents of positive resists. In designing materials for lithography using deep UV, it had been believed advantageous to use m/p-cresol novolak resins containing p-cresol as a major component (Materials for Microlithography, P340ACS Symposium Series 266, ACS, 1984). This belief comes from the fact that, comparison of m/p-cresol novolak resins shows cresol novolak resins composed of 100% p-cresol or mainly p-cresol exhibit less absorption in the deep UV than the popularly used m/p-cresol novolak resins.
Attempts were also made to overcome the problems of the prior art by changing the m/p ratio, the ester ratio of the light-sensitive substance, and the amount of light-sensitive substance added in order to alter the optical density.